The ornamental design for an instrument, as shown and described.
FIG. 1 is a perspective view taken from a top, front, left side of the instrument;
FIG. 2 is a front view thereof;
FIG. 3 is a right side elevation view thereof;
FIG. 4 is a left side elevation view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a back view thereof; and,
FIG. 7 is a bottom view thereof.
Throughout the Figures, the dash-dotted lines represent the bounds of the claim, and all other broken lines illustrate unclaimed portions of the instrument. None of the broken lines form a part of the claimed design.